In a permanent-magnet synchronous machine, a magnetic field is generated using permanent magnets installed in the machine's rotor. The permanent magnets are either fitted on the surface of the rotor facing the air gap, or the permanent magnets are embedded into the rotor's magnetic core. The magnetic core is most often made of ferromagnetic iron sheets assembled into a sheet core extending throughout the rotor's length. The design criterion for dimensioning the permanent magnets and sheet core is the excitation power required by the electrical machine. The mechanical structure and the fastening of different components are affected by factors such as the forces imposed on them and the size of the electrical machine.
A permanent magnet is a component manufactured from a magnetically hard material and is able to retain its magnetism permanently after magnetization. Permanent magnets are manufactured, for example, from an AlNiCo mixture that includes aluminum, nickel, cobalt and steel, or from ceramic materials or rare earth metals.
As permanent magnets are embedded into the magnetic core of a rotor assembled from uniform sheets in a characteristic manner, thin necks will be generated on both sides of the permanent magnet, supporting the pole piece. As the rotor rotates at a non-standard speed or varying speed, the thin necks bear the load caused by centrifugal forces generated by the pole and magnets, and an alternating load is directed at the thin necks. The alternating load causes fatigue, and the allowed fatigue stresses are significantly lower than the allowed static stresses. In addition, some of the flux generated by the magnets is lost because of the stray flux short-circuited by the thin necks.
Centrifugal forces also tend to cause transitions in the pole. In order to prevent any transitions, the necks must be able to maintain tight compression.
A characteristic method of attaching permanent magnets to the rotor surface facing the air gap is to glue the permanent magnets to the rotor surface and install a supporting binding made of carbon fiber around the rotor. Disadvantages of this solution include a decrease in the mechanical air gap between the stator and rotor, which may cause problems in installation or damage the binding during installation or when the rotor rotates, or an increase in the magnetic circuit's air gap if the mechanical air gap is not to be reduced. This will increase wear in the magnetic material.